This invention relates generally to a method and apparatus for forming a charge pattern on an imaging surface and more particularly to a method for forming a charge pattern using an ink jet apparatus.
The formation of charge patterns an imaging surfaces is well known, especially in the xerographic arts. In xerographic processes, typified by the Carlson process originally disclosed in U.S. Pat. No. 2,297,691, a photoconductive insulating imaging surface is first uniformly electrostatically charged. The charged surface is then exposed to imagewise radiation to which the surface is sensitive, such as light, and the charge in the radiation-struck area is dissipated. The charge remains on the imaging surface in the non-radiation-struck areas to form a charge pattern. Such a charge pattern is commonly referred to as an electrostatic latent image.
The uniform charging of the imaging surface is typically accomplished, for example, by the method disclosed in U.S. Pat. No. 2,588,699 to Carlson which involves the use of an ion producing filament or filament arrays operating on corona discharge principles. However, such uniform charging can be accomplished by contacting the surface with a charging electrode as disclosed in U.S. Pat. No. 2,774,921 or with a charging brush as disclosed in U.S. Pat. No. 3,146,385. Other methods of uniform charging include the use of pin electrode arrays as disclosed in U.S. Pat. Nos. 2,934,650; 3,649,830; 3,655,966 and 3,689,767.
An alternative method of forming a charge pattern by uniformly charging an imaging surface with a charging electrode through a mask is described by Gundlach in U.S. Pat. No. 2,912,586.
The charge pattern thus formed is oftentimes made visible or developed with marking material by development processes well known in the xerographic arts. A typical such development process is described by Carlson in U.S. Pat. No. 2,297,691.
A method for forming a charge pattern on an imaging surface alternative to the method of charging through a mask is desirable. Such an alternative method which can produce charge patterns responsive to electrical input from, for example, a computer or a remote optical scanning device, is especially desirable.
Ink jets are well known in the art as a means for direct writing on an imaging surface. Ink jets normally project a dyed or pigmented liquid onto an imaging surface responsive to electrical or mechanical control. Various types of ink jets are known. Some produce a stream of liquid which is broken into droplets by ultrasonic vibration as the stream emerges from a nozzle. Other ink jets rely on an electric field to draw droplets from the open end of a small nozzle. Still others use pulsing mechanisms to squirt droplets from an orifice.
In many of the known ink jets direct writing systems the droplets are charged as they exit the ink jet orifice. The droplets are most often charged so that their trajectory from the ink jet orifice to the imaging surface can be controlled by electrons placed along the trajectory. The electrodes are usually electrically controlled, for example, by computer output or by remote optical scanning of an original image.
Typical examples of direct writing with charged, colored particles from ink jets are shown in U.S. Pat. Nos. 3,596,275 to Sweet and 3,852,772 to Hecht et al. Sweet discloses deflection of charged droplets to create an image pattern on a surface. Sweet shows the use of electrodes to deflect the droplets. Hecht et al shows uncharged droplets impinging on a receiver sheet while selectively charged droplets are deflected.
It is to be noted that the use of deflecting electrodes requires that the ink jet orifice be spaced a distance from the imaging surface sufficient to permit the electrode to have an effect on the trajectory of the ink droplet. Such spacing is sometimes undesirable in compact arrangements of apparatus.
The direct writing ink jets of the prior art generally make use of dyed or pigmented liquids. Such liquids are known to dry in the ink jet orifice when not in frequent use, causing clogging problems. One attempt to overcome the clogging problem common to most direct writing ink jets centers around increasing the orifice size. However, increasing the orifice size undesirably reduces the resolution of the directly written image. The larger orifice size results in the image being written with large droplets which are capable of less image definition.
A method and apparatus for forming a charge pattern on an insulating surface using a stream of ionized fluid, such as gas, is disclosed in U.S. Pat. No. 3,715,762 to Magill et al. However, the method so disclosed requires a second non-ionized, fluid stream to deflect the ionized stream when no charge is desired on the insulating surface. Ionized gases are known to be unstable and difficult to control with accuracy, and there is no way in such a system to visibly inspect the charge pattern prior to development, if desired.